Earth runs on massive amounts of heat, enough to melt iron in the core and create our magnetic field, enough to power the constant movement of plate tectonics. Where all this heat comes from is a mystery...but we're getting closer.

Forty-four trillion watts worth of heat is constantly flowing out of the Earth's interior into space, but it's only relatively recently that we've been able to say with any certain what is creating all that heat. We do know that a major factor in all this heat creation is the radioactive decay of elements like uranium, thorium, and potassium, but the latest research indicates that that only accounts for about half of our planet's heat. So what's creating all the rest?

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The innermost reaches of our planet are beyond the bounds of direct observation, so we're forced to make a few key assumptions. Geologists generally favor what's known as the Bulk Silicate Earth model, or BSE, which holds that all the elements in the crust and mantle are lithophiles, or "rock-loving elements", while the elements in the core are siderophiles, or elements that like to be with iron. Since all the radioactive elements are lithophiles, they must be restricted to the crust and mantle.

The latest research from the Japan-based KamLAND collaboration, which uses neutrinos to track heat signatures from within the Earth, says that uranium 238 decay accounts for about eight terawatts of the planet's heat, thorium 232 gives us another eight, and potassium 40 another four. That still leaves more than half the planet's heat energy unaccounted for, including all the heat created in the core. Another three terawatts is probably created by all the other radioactive elements combined, but that still leaves a ton of heat that remains unaccounted for.

KamLAND member Stuart Freedman of the Berkeley Lab explains what these latest findings mean, and where they're headed next:

"One thing we can say with near certainty is that radioactive decay alone is not enough to account for Earth's heat energy. Whether the rest is primordial heat or comes from some other source is an unanswered question. This is what's called an inverse problem, where you have a lot of information but also a lot of complicated inputs and variables. Sorting those out to arrive at the best explanation among many requires multiple sources of data."

For now, we're left to guess what the other sources of heat might be. One possibility is that a huge amount of heat was trapped inside the planet as the Earth formed billions of years ago. But for now, we're left to wonder where all that other energy is coming from...but hey, at least we're now halfway there.